With the rapid advancement of signal processing technologies, digital audio technologies has emerged as the preferred method for transferring and storing audio information. Digital audio comprises analog audio signals that are represented in a numerical form, such as binary integers. Conversion from an analog signal to a numerical form typically involves sampling the analog signal at various times and converting the sampled portions of the analog signal to numerical values which are usually represented in binary form and often referred to as audio data.
Digitizing analog audio signals has its advantages in that it makes managing the audio signals easier. For example, audio represented in binary form makes it possible to transfer audio signals over digital mediums, such as the Internet. Moreover, binary digitized audio makes it possible to store, play, edit and otherwise manipulate the audio signals on, e.g., a computer system.
One coding technique that is widely used to generate digital audio from analog audio signals is Pulse-code Modulation (PCM). PCM is a wave-form coding technique that involves converting analog signals into a series of numerical values that represent amplitudes of the analog signals at various times. A PCM representation of an analog signal is typically generated by measuring (sampling) an instantaneous amplitude of the analog signal, and quantizing the result. Quantization involves converting each sampled value into a discrete value that is usually represented in numerical form, such as a binary integer. Quantization may be accomplished using specially programmed analog-to-digital converters which are configured to sample the amplitude of an analog signal at regular intervals and convert the sampled amplitude to a binary value. The quantized samples may then be stored as audio data in a data file contained on a digital medium, such as a disk, for later playback. Playback may be performed by converting the quantized samples to an analog signal using specially programmed digital-to-analog converters. A file format that is often used to store PCM encoded data on a computer system is the well-known Waveform Audio Format (WAV). Other file formats used to store PCM encoded data in various forms include the well-known Moving Picture Experts Group (MPEG)—1 Audio Layer 3 (MP3) format.
Some computer-based applications have been specially designed to enable users to edit PCM encoded audio data contained in files, such as WAV files and MP3 files. Often these applications present the audio data to the a user in the form of different type of displays, including waveform displays and spectral displays. Waveform displays typically depict a sinusoidal shape of the analog signal as represented by the audio data. Spectral displays, on the other hand, typically depict spectral aspects of the analog signal as represented by the audio data. These aspects may include phase, pan position and frequency of the analog signal.
One common form of spectral display is a frequency spectral display. A frequency spectral display displays data as a function of time, frequency and amplitude where time may be displayed on a horizontal axis, frequency may be displayed on a vertical axis and amplitude may be displayed in the form of a color or intensity. Here, audio data may be analyzed using, e.g., Fast Fourier Transforms (FFTs), to identify various frequency components (e.g., audio frequencies) represented in the audio data. The identified frequency components may then used to render the frequency spectral display.